Telephone station circuit



May 15, 1945- K. s. JOHNSON 2,375,791 4 TELEPHONE STATION CIRCUIT Filed March 9. 1943 INVENTGR K S. JOHNSON v I jimg ier Patented May 15, 1945 UNITED STATES PAT ENT OFFICE TELEPHONE STATION cmcm'r Kenneth S. Johnson, South range,N. J., assignor to Bell Telephone Laboratories, Incorporated,

New York, N. Y a corporation of New York Application March 9, 1943, SeriaLNo. 478,499

9 Claims. (Cl. 179-81) This invention relates to anti-sidetone subscriber station circuits and, more particularly, to an arrangement for controllin the efiect of room noise in the telephone receiver of such circuits.

In an ideal anti-sidetone telephone substation V circuit no current would be produced in the re- 1 ceiver of the circuit during transmitting out of the circuit, that is, during agitation of the transmitter of the eircuitby the vocal output of the user thereof, nor, during receiving, would any interfering alternating current be produced in the receiver as aresult of room noises effective to agitate the transmitter. This result generally is predicated upon a circuit arrangement in-which the componentatogether with the telephone line to which the station circuit is connected, constitute the arms of a bridge proportioned to bebalanced with respect to the currents generated by .the transmitter, the receiver being connected to the points of equal potential. In practice, this condition is not realized for a number of reasons. The station circuit may be designed for a particular or an average line and is in fact used with.

a different line or with one that does not con form to the characteristics of the average line. Hence the condition of balance is not attained. In a station circuit includin a transformer or induction coil; a receiver, a transmitter, and a line balancing network, the practically. attainable ter in such a circuit should be replaced by a more efficient instrument sidetone might rise beyond the tolerable level. On the other hand, the un balance between the line balancing network and line impedances in a particular installation might produce a similar end effect. Although such increase in the sidetone level is objectionable during transmitting out offthe station circuit, effective use of the station circuit for transmittin purposes is not necessarily precluded. The increased effect in the receiver, however, during receiving, of room noiseseffective on the transmitter may seriously impair the effectiveness of the station circuit for listeningpurposes inthat the weak telephone currents incoming from the telephone line may bes'o interfered with as to be masked or rendered unintelligible in part or as a whole.

An object of this invention is to eliminate room noise efiects in the receiver of an anti-sidetone subscriber station circuit duringreceiving at the station.

Variousexpedients to minimize or to eliminate room noise efiects in the receiver of a station circuit during receiving have been proposed. These have usually involved: the provision of means to short-circuit the transmitter alone, 01' to short-circuit the transmitter and a local battery therefor, or to open the transmitter circuit.

The first-mentioned eXpedients have the drawback of wasting local battery current, and the last that of roducing a click in the receiver and in the line or at the distant station upon the openin and subsequent closing of the circuit.

'In accordance with the invention, an antisidetone subscriber station circuit may comprise a receiver, a transmitter, a multiwinding induction coil and a network for balancing the imped ance of thetelephone line connecting the station circuit to another station circuit or to a telephone exchange or central office. The transmitter and one winding of the induction coil may be connected in a local series circuit including a local battery for the transmitter. The receiver may be connected in series with a second winding of the induction coil across a pair of lineterminals to .whichthe telephone line would be connected. A

third winding of the induction coil and the line balancing network may be connected in shunt with the receiver. The network may be a resistance included in the third winding. A normally-open switchis connected across the second winding, and is adapted to be operated to. closed condition by the station user during receiving at the station. Closure of the switch short-circuits the winding across which it is connected and thereby effectively short-circuits the 3 other winding of the induction coil, and simultaneously connects the telephone receiver directly across the line terminals. Hence, room noises effective on the transmitter are precluded from producing. interfering currents in the receiver. At the same time, the transmitter local circuit remains unchanged, that is, the direct current flowing in the transmitter 01' its associated induction coil winding is not interrupted or opened, nor is either the transmitter or itsserially-connected induction coil winding directly short-circuited. As a consequence, these possible origins of receiver click or of wasteful drain on the local battery are eliminated.

A more complete understanding of the invention will be obtained from the detailed description that follows, taken with reference to the attached drawing, the single figure of which illustrates an embodiment; of the invention.

The telephone substation circuit shown'in the drawing comprises a pair of line terminals I, 2

for connection to a telephone line 3 leading to an:

other telephone station circuit or to a telephonecr exchange or central office; a hand telephoneor handset 4 comprising a transmi'tterwT}for example, of the variable resistance type; a'receiver R, and a finger-operated push-button'orswitch I mounted in the handle portion-5 of .-th :handset;:

a three-winding transformer or induction coil 8 comprising windings N1, N2 and N3 andaline balancing-network .X, which may comprise a .re-- sistanceincluded in the winding. N3 ;v and a spring assembly 9 comprising circuit-controlling.,contact pair's I',-:I|.- The latterareshown closed,'

the condition they assume when the handset is removed from.the. .-cradle.v plunger support .12.

With the handset on thesupport, the plunger 'op-T crates zonthe-springassemblytoopen the..con.- tact pairs II], II. A serially-connected station.

signal. ororinger.v l3 and condenser. M may. be

connected acrosstheline. terminals. A condenser l .may be connectedin series with thereceiver. to prevent. flowofline direct current through the receiver....A.localisource-of current, for example, a-battery .46, isprovided at the station. to supply; current iorthe transmitter.

When .the handset is removed from its support, closure of themcontact pair It .icompletes alocal. seriescircuit for the transmitter that comprises.

the transmitter, the battery, (winding N1 and contactpair I05 Closure .of contactpairll completes. a'series. circuitfor'the receiver across the line terminals comprising terminal. I winding N2, the -receiver, condenser. l5.,.contact pair ll, termi-.

nal 1-2. .The windingsNs -and-balancing.network. H

X remain connected acrossthe. series-connected receiveriand condenser: l 5.; The circuits just described constitute: a clocal battery anti-sidetone telephone station.circui t of. the type disclosed in G. A. Campbell U. S; Patent.1,254,472,bf January 22 1918; particularly with. reference to. Figs; 1 and. 4 thereof. Iftheline -3 hasthe characterise tics of thepartic-ular lineor the average line. for.

whichv the station circuit was. designed, and .if the transmitter and the receiver have the. characteristics assumedin. such .design,\.sidetone.in

thereceiver. duringtransmitting. and .re'ceiving through theline terminal .will .be absent or, negli gible. If, howeven-in thev particular system or installation the impedance Of the lineand that-of. the. line balancing network are markedly disproportionate, or the transmitter. or receiver is. .of

considerably higher efiiciency than initiallycontemplated, the sidetonedeveloped in thereceiver during transmittingiand(receiving may. be ape. preciable; .If not of. a level to produce .singing,l

sidetone. during transmitting..can be tolerated.

since itdOes/Anot affectintellig'ibility of the out-. going .telephone-currente. that. is, the currents. generated .whenzthe. transmitter is agitated-by the. vocal .output of the user. During/receiving,

however, the. sidetone .inthe receiver resulting from alternating. currents generated .byagitation of .the. transmitter .byroommnoisesmay vbe suf.-- ficient. to. interfere. withithe weakincoming .-tele.-. phone currents to such'an extent as to mask the tact |8,-the winding N2 is short-circuited. It can be shown that inan ideal transformer shortcircuitingone =;winding thereof for which the number ofturns is not zero, short-circuits all the windings of the "transformer. Short-circuiting windingNz, therefore, effectively short-circuits the winding N1 and the winding N3, and even though current may be generated in the transmittensuch currents are notreproducedin either winding. N2 'or N3; At the time/the windingNz is...

short-circuited, the seriallysconnected..receiven and. .condenser .l will .be. connected directly. .to. the terminals I 2 and the incoming telephone.

current will be effective .inthe receiver without interference from sidetone.resulting.,from roonr..

noises .that may, be agitatingwthe. transmitter..,. During. transmitting..-the manual pressure oncthen switch 1 is released to. remove-the. short-circuit onthe winding N. .Since there isno actual short-1 Y circuit placed across. the winding N1 or. the; trans... 1 mitter, and since. the. .local -.battery..circuit rea mains unafiected,.ithere is no. abrupt. flux change, in thwinding. Ni'v that might evidence. .itself. as.v

an objectionable; "click? at the beginning-"and.

the end of, thereceiving .or listening .period,..nor

is the normal battery .drain .altered.

Although. the invention has. .been. illustrated 1 a scope evidenced by the appended claims.

What. isolaimed is a.

1.. In combination with a telephone substation. circuitfadapted to be connected lwitha telephone 45'" ,withjreference to-a single embodiment,it will be understood that. itisnot. limited thereto .butris of.

line and including a multiwinding transformer and a transmitter connected in a, localcircuit' withone ofthe windings of the transformer, a-.-

source or direct. current-ion the transmitter,.and

means :to effectively. shortcircuit the. transformer.

windings duringnon-talk'ing periods, while permitting direct .current.flow to be maintained substantially unchangedxin the. transmittem 2. In combination .withia telephone substation 1 circuit. adapted to be-connectedewithta telephone line "andmcomprising. a multiwindin .transformer and a transmitter connected .in a -l0cal:-circ1lit-in-.

eluding. one of. thetr'ansformerv windings, a source of directcurrent for the.transmitter;.andflmeanss connected across another. transformer Winding' to, efiectively short-circuit the transformer wind-L ings during.non-talkingperiods while. permitting.

direct current flow .to ebelmaintainedNsubStane tially unchanged in the .transmitterv I 3. A telephone. substation circuit adapted to beconnectedwitha telephonelinesaid circuitcome prising. a. .multiwinding. transformer, a transmit ter,. a .source. .ofdirect current..for..said transemitter, said transmitter, current,- source. and..a

winding. of. the transformer ..being..,connected in a xlocaLcircuit, andmmeans to effectively ,short.-.

circuit. the windings. of the. transformer during,

non-talking periods, .while permittinggdirectcur rent. flow totlbe maintained". substantially; un.-.

changedinthetransmitter;:

4. A telephone substation circuit adapted to be connected with a telephone line, said circuit comprising a multiwinding transformer, a transmitacross said line terminals, a local transmitter circuit inductively coupled to said receiving cir cuit and comprising a transmitter, a battery and a second induction coil winding, and means for disabling the induction coil when receiving at said substation, for transmission of alternating current from thel ocal transmitter circuit to said receiver and for connecting said receiver directly across the line terminals.

6. A telephone substation circuit adapted to be connected to a telephone line, comprising line terminals, a receiving circuit including a receiver and an induction coil winding serially connected across said line terminals, a local transmitter circuit inductively coupled to said receiving circuit and comprising a transmitter, a battery and a second induction coil winding, and means for short-circuiting the first-mentioned winding and connecting said receiver directly across the line terminals.

'7. A telephone substation circuit adapted to be connected with a telephone line, comprising line terminals, a multiwinding induction coil, a transmitter, a receiver, a. local circuit for said transmitter including said transmitter and one winding of the induction coil, a line circuitcomprising a second winding of said induction coil and said receiver connected across said line terminals, andmeans for short-circuitingsaid second coil winding during receiving at the substation,

whereby alternating currents generated in the transmitter are precluded frombeing introduced into the receiver during receiving.

8. A telephone substation circuit adapted to be connected with a telephone line, comprising a handset including a transmitter, a receiver and a manually-operable normally-open switch, a multiwinding induction coil and a line balancing network, a first circuit including said transmitter and one winding of said induction coil, a second circuit including said receiver and a second winding of the induction coil, and a third circuit including a third winding of the induction coil and the line balancing network, said circuits being inductively coupled through said windings, said switch beingconnected across one of said windings and being adapted for closure during receiving at the substation to short-circuit the windings of the induction coil, whereby alternating currents generated in the transmitter during receivingare precluded from appearing in the receiver. i

9. A telephone substation circuit adapted to be connected with a telephone line, comprising line terminals, a handset including a transmitter, a receiver and a manually-operable normally-open switch, a battery, a multiwinding induction coil and a line balancing networlga local circuit including saidtransmitter, said battery and one winding of said induction coil, a second circuit including said receiver and a second winding of the induction coil connected across said line terminals, and a third circuit including a third winding of the induction coil and the line balancing network connected in shunt with the receiver, said circuits being inductively connected through said windings, and said switch being connected across the second winding and adapted for closure during receiving at the substation to short circuit said second winding and to connect the receiver directly across the line termi-, nals, whereby alternating currents generated in the transmitter during receiving are precluded from appearing in the receiver.

KENNETH S. JOHNSON. 

